While not universal, many segments of the world's population suffer from the lack of an adequate water supply at some time during the year. When this occurs, crops fail for want of irrigation and major health problems arise, both from dehydration and from the compulsive, albeit imprudent, use of contaminated water supplies to satisfy human thirst.
Throughout history efforts have been directed to the creation of back-up or auxiliary water supply sources such as reservoirs or tanks, strategically placed dams in river beds, canals and the like. However, as is well known each has failed to totally resolve the problem and as a result, a need still exists for insightful means and methods to manage nature's water supply in such a way that mankind can have a ready source of usable water throughout the year regardless of general climate, terrain or the idiosyncrasies of local weather patterns.
One area which typifies those geographical areas which have heretofore been subject to great heat and highly cyclical and frequently unpredictable rainfall is the Sonora desert which extends from the Southwest United States into Northern Mexico and includes Phoenix, Arizona, the ninth largest city inn the U.S.
In the Phoenix Metropolitan area, the Salt River Project (SRP) is the largest raw water purveyor, founded basically to serve agricultural users and communities which do not have their own water supply. SRP delivers primarily through canals but, in addition, has approximately 250 wells to supplement its extensive surface water supply during dry periods and peak demand time. It is believed that these wells, which are located adjacent to SRP's canal conveyance system, could be further used for artificial ground water recharge if means could be developed to filter and treat the canal water to render it compatible with the wells and thereafter introduce that compatible water into the wells during idle ground water pumping periods.
A pilot project was commenced in 1991 to evaluate the feasibility of using the SRP wells for artificial ground water recharge in the alluvial aquifer of the Salt River Valley. A test injection station was established in a well site which was found to possess all the desirable characteristics, including a monitoring well. An innovative injection system was installed and evaluated. This system used a rotating microscreen for filtration, and chlorination of canal water prior to its injection into the well (Gorey et al. 1989, Lluria et al. 1991). Although this system proved to be effective in reducing particulates, it was incapable of providing an adequate flow rate, was costly to erect and operate, and required considerable maintenance. The drawbacks encountered in this approach motivated SRP to announce a need for alternative solutions to the problem. The filtration system of the present invention was devised in response to that announcement.
The use of idle production wells for ground water recharge is an attractive idea. However, such a well must be characterized by favorable geohydrology and ground water quality characteristics. Furthermore, the recharge water must be compatible with the well or else irreversible well damage may result. Three mechanisms exist that can plug and permanently damage a well: chemical, entrained air, and suspended particulates. Chemical plugging occurs when the water chemistry of the water used for recharge is significantly different than that of the water in the aquifer because it causes an ionic/chemical reaction to occur which closes up the pores of the well, and reduces its permeability. Air entrainment also adversely affects a well because air bubbles, driven by capillary action, become lodged in the formation and reduce its permeability. Indeed, the damage from air bubbles can be so extensive that well redevelopment, that is, pumping all water out of the well, may not be able to reverse the damage. Suspended particulate (organic and mineral) plugging is probably the major cause of reduction of recharge rates in injection wells because the clogging of the intergranular pores in medium and fine grained sediment aquifers substantially reduces their permeability. If the suspended solids concentration is large and the time of injection with this type of water is considerable, permanent damage to the well may also result. Well redevelopment, using pumping and surging, is the only method presently known which can improve well recharge performance. If the source for well recharge is untreated water, frequent redevelopment will be required at very short time intervals, elevating the cost of the recharge operation and reducing the long-term injection rate.
In dealing with canal water, mineral particulates did not present a problem. On the other hand, the canal water was rich in organics, primarily algae, and that presented a major challenge to the filtration system.
The desiderata leading to the present invention was the quest for a filtration system which has the ability to remove large particulates and organics, provide high flows, keep air from being entrained, and which would be relatively inexpensive to build and operate.
In our pursuit, it was learned that filters which work well in the water treatment business are not necessarily feasible for recharge. For instance, the centrifugal separator, an excellent means to remove sand and heavy particulates, could not be used for water recharge because the suspended particulates in canal water are primarily organic and have a density close to that of water. As a result, centrifugal forces cannot differentiate between the organic particulates and water.
Further, the cost of the filter must be relatively low since present economic incentives for recharge are modest. This also means that the cost to build and operate the system must be low, and to be cost-effective, the system must be able to produce large flows. It is toward accomplishment of these goals that the present invention is directed.
Of course, there are also many prior art patents which deal with fluid filtration systems and their associated backwash or filter cleaning mechanisms developed to attempt to improve filtering efficiency. Among such disclosures are Vandercook (U.S. Pat. No. 1,139,825) who discloses a filter system in which a rotating nozzle assembly is used to spray a solid-liquid mixture against a horizontal filter. This spraying action not only facilitates the filtration, but also helps to keep the filter free from excessive buildup of unfiltered solids. A backwash process is also disclosed in which either water or air is forced up through the filter from below and coacts with a stream of water or air from the rotating nozzle assembly to dislodge debris from the filter surface.
Morino (U.S. Pat. No. 2,851,164) discloses a swimming pool filter in which a filter element is suspended in concentrical relationship about a fixed spray head which consists of a vertical tube having a plurality of holes disposed therein. Whether filtering or backwashing, incoming water is flowed into and through the fixed spray head. A valve is provided to increase the flow pressure during backwashing to create a "jet" to clean the filter element.
Snyder (U.S. Pat. No. 3,193,103) discloses a submersible pond filter having a cylindrical metal mesh strainer which contains a cleaning mechanism comprising a plurality of flexible, elastomeric hoses disposed therewithin. Fluids such as water, air, or both are forced under pressure into the elastomeric hoses which causes them to whip and flail about and direct a stream of fluid against the inner surface of the filter to dislodge debris therefrom.
Everroad (U.S. Pat. No. 3,236,249) discloses a free-standing filter cleaning device for cleaning individual cylindrical filters which are imported into the cleaning device. The device also includes a rotatable spray head arrangement comprising a cylindrical inner spray standard having a plurality of spray heads formed thereon, and two outer spray pipes, each having a plurality of spray heads which face radially inwardly. The inner sprays deliver a cleaning fluid radially against the interior of the filter, while the outer sprays spray the exterior. The outer pipe spray heads are positioned at a slight lateral angle thereby causing the entire spray head arrangement to rotate about its cylindrical axis during the cleaning operation in response to the "jet propulsion" created thereby.
Knieriem (U.S. Pat. No. 3,337,052) teaches a filter apparatus comprising an outer housing having a detachable top, and an inner cartridge assembly having a sintered filter element as a substantial part of its side wall. Fluid to be filtered flows into the housing, through the filter element on entering the cartridge and then flows out of the housing. The filter element is a porous, electrically conductive, sintered material that has a substantial electrical resistance. To clean this filter of clogged debris, an electrical current is passed through the filter element to heat the filter and burn any material caught on the filter. Gas purge means are then provided to blow the resulting ash from the assembly.
MacFarlane (U.S. Pat. No. 4,759,846) discloses a rotatable filter drum with a permeable filter element. The fluid to be filtered is passed through the filter element to the interior of the filter drum from which the filtrate then exits. To backwash the filter element, a plurality of hollow backwash arms are disposed outside of and operably bear against the filter element to provide a backwash seal. The backwash arms are preferably positioned so that they will sweep the entire surface of the filter element during one full rotation of the filter drum.
In the use of this device, backwashing and filtering can occur simultaneously. As the filter drum rotates, most of the fluid flows through the filter and out the filtrate outlet. However, a small percentage of filtrate is simultaneously forced back through the filter element into the backwash arms. A lower pressure is preferably maintained in the backwash arms to suction the limited back flow into the arms. Contaminants are dislodged by the back flow and carried out through the backwash arms. Rotating spray arms that sweep the inside surface of a filter drum may also be used to help loosen the debris.
Wilkins, et al. (U.S. Pat. No. 4,822,486) teaches a cylindrical, rotatable self-cleaning strainer or filter having a fixed nozzle structure mounted within the filter. The nozzle structure is supplied with water from an independent source and the water is forcefully discharged against the filter to dislodge trapped debris from the outside of the filter.
As is readily apparent, none of the prior art teaches a filtration system which achieves all of the aforestated goals. It is toward the realization of those goals that the present invention is directed.